As is well known, the effluents from industrial plants which produce or use metallic mercury and/or mercury salts, (as in the production of chloro-alkalis with mercury cathode cells; the production of derivatives and metallic mercury; the recovery of mercury from processing wastes, etc.) and which are discharged into public waters, pollute the water with resulting biological damage.
In general, the environmental laws in force throughout the world impose such tight limits on the concentration of pollutants which may be present in water that complicated, expensive plants are required for the purification of the effluents which contain mercury and mercury derivatives.
As is also known, the mercury ion, particularly in solutions containing chlorides, forms a variety of compounds such as, for example, [HgCl.sub.4 ]--, HgClOH, and HgCl.sub.2.nH.sub.2 O, all of which are more or less soluble even in alkaline media. Consequently, in purifying effluents polluted by mercury and mercury derivatives it is always necessary to deal with a wide range of more or less soluble compounds, and this makes the operation still more complicated because, at least in some instances, it is necessary to use several different purifying agents.
The present chemical purifying processes, (for example, those using hydrazine, sodium sulfide, sodium boro-hydride, reducing metals, etc) almost always lead to an insoluble product which is hardly separable from water, so that complicated and expensive decanting operations are required, followed by filtering with the use of filtering aids.
In other of the known processes (for example processes involving the use of resins), the inevitable presence of muds and inert products which exist naturally in the water to be treated requires pre-treatment of the water for the removal of the naturally occurring mud and inert products by filtering or decanting upstream of the purification plant.